Unifier class: `unify` method returns duplicated patches

Unifier class: `unify` method returns a list containing duplicated patches in the case of conflicts containing equal patches (same path).

e.g.
```
patch1 = ('remove', '', [('a', 'b')])
patch2 = ('remove', '', [('a', 'b')])
conflicts = Conflict(patch1, patch2)
conflicts.take = 'f' # can be 's' too
```

The result of calling `u.unify([patch1], [patch2], [conflicts])` will be `[patch1, patch2]` instead of `[patch1]`. This particular case can lead to `KeyError`s in case of calling `patch`, as it will try to remove the same key twice. The second time it fails as the key has already been deleted.